In general, in accordance with a write command from a host apparatus, a magnetic disk device writes data of a data length specified in the write command (in other words, write data) to a disk. However, before the writing of the write data is completed, a power supply (more specifically, a main power supply) to the magnetic disk device may be interrupted. In this case, part of the write data (more specifically, data not yet written to the disk) may be lost.
Thus, recent magnetic disk devices have a power loss protection (PLP) function to secure write data even when the power supply is interrupted. The PLP function refers to a function to save data not yet written to the disk to a nonvolatile cache at a high speed when the power supply is interrupted. This saving operation is performed using power temporarily supplied by a backup power supply.
However, the amount of write data secured by the PLP function depends on, for example, the capacity of the nonvolatile cache. In other words, the amount of write data secured by the PLP function is limited. Thus, in general, after all of the write data is written to an area on the disk specified in the write command, the magnetic disk device reports a status for the execution of the write command (for example, a good status indicative of write completion) to a host. In this case, the host needs to wait to issue the next command to the magnetic disk device until the host receives the report of the status from the magnetic disk device. This may degrade the performance of the magnetic disk device.